These plans are confusing, but they're made much simpler if you read this informative table, in the first post on a forum at MacTalk:
http://forums.mactalk.com.au/47/52261-official-optus-pricing-information.html
Full rundown of total costs, minutes per plan, etc. Quick summary:
- The "yes" Cap plans are better if you mostly want voice
- The "yes" plans are better if you mostly want data
- The absolute cheapest total monthly price is AU$40/month over 24 months but you only get 42 minutes of calls and 100MB of data
- The cheapest way to own an iPhone outright is to buy prepaid ($729) then buy a $30 recharge followed by a $50 recharge (you need to recharge $80 to get free unlocking or pay that much instead) and since you get bonus credit and data in your first month (plus unlimited data until the end of August) you might as well use it.
- It's possible to go on a long-term low-end plan for very slightly more than the outright price but you'll likely need more than the minutes included (13/month).
- Australians (and Europeans) don't pay for incoming calls, so these "minutes" are for outgoing calls only.
As others have mentioned, the iPhone is on most of the networks. However, there are several resellers (eg. Virgin, who use the Optus network) who aren't selling the iPhone but have decent data deals (1GB for $15) so there is an argument for unlocking a prepaid handset.
Vodafone are yet to release pricing but I'd suspect them to be similar; perhaps cheaper as they have no Wi-Fi network to bundle in.
Telstra have released price points ($30, $80, $100 plans + $279 to $0 cost) + but no information on included data or call rates; their existing deals at $30 and $80 include 5MB or 10MB of data and it remains to be seen if that will change for the iPhone.
Hope that helps.